The present invention relates to a semiconductor optical functional device, and more particularly, to a semiconductor optical functional device adapted for use as a polarization-independent optical switch which can operate in response to both TE and TM mode light. There have recently been proposed semiconductor optical switches in which waveguide layers, for example, are formed having a quantum well structure for improved performance.
In general, this quantum well structure is formed by stacking a plurality of rectangular potential quantum wells for use as fundamental units. Each quantum well is composed of a thin semiconductor with a thickness substantially equal to the de Brogilie wavelength of electrons, which is inserted between the two semiconductors whose band gap energy are greater than that of the thin semiconductor.
In the quantum well as the fundamental unit hereinafter referred to as single quantum well), the inserted semiconductor and the holding semiconductors form a steplike quantum confined potential, which discontinues at the interface planes between the semiconductor and the holding semiconductors.
In the quantum well structure, the energy levels for electrons and holes are quantized, and the electrons and holes are confined within a very thin region. Even at room temperature, therefore, clear sharp light absorption peak is manifested due to the production of excitons which are each formed of an electron and a hole in a binding state.
If an electric field is applied perpendicular to the quantum well plane moreover, the position of the excitonic absorption peak is shifted to the longer wavelength side without broadening its sharp peak, which is called the quantum confined Stark effect. In the quantum well structure, appear a great change in absorption coefficient and a subsequent change in refractive index for the light of wavelengths in the vicinity of the absorption edge.
Accordingly, the light absorption in the semiconductor can be increased or decreased by applying the electric field perpendicular to the well plane, for the light with wavelengths between the absorption wavelengths before (shorter) and after (longer) the application of the electric field. Thus, switching operation can be performed.
In the rectangular quantum well structure described above, degeneracy in the energy levels of heavy and light holes, which is characteristic of bulk semiconductors, is eliminated, and the quantization energy levels for the individual holes separately exist. In the aforementioned excitonic absorption at the absorption edge, therefore, an absorption peak corresponding to an exciton formation from a ground-level electron and a ground-level heavy hole (hereinafter referred to as 1e-1hh transition) develops on the longer wavelength side, while an absorption peak corresponding to an exciton formation from a ground-level electron and a ground-level light hole (hereinafter referred to as 1e-11h transition) develops on the shorter wavelength side.
If an electric field is applied perpendicular to the quantum well plane in this state, the absorption peak corresponding to the 1e-1hh transition greatly shifts to the longer wavelength side, while the absorption peak corresponding to the 1e-11h transition which is originally situated on the shorter wavelength side, much less shifts to the longer wavelength side.
Since the energy shift attributable to the quantum confined Stark effect is substantially proportional to the effective mass of a particle associated with the transition, the shift in excitonic absorption energy corresponding to the 1e-1hh transition, which includes a heavy hole with a mass heavier than light hole, is large.
Accordingly, the changes in the absorption coefficient and refractive index in the vicinity of the absorption edge practically depend on the excitonic absorption corresponding to the 1e-1h transition. Therefore, the switching operation using light with wavelengths near absorption edge is regulated by the excitonic absorption corresponding to the 1e-1hh transition.
The 1e-11h transition interacts with both a light having a component whole photoelectric field is parallel to the quantum well plane (hereinafter referred to as TE mode light) and a light having a component whose photoelectric field is perpendicular to the quantum well plane (hereinafter referred to as TM mode light). On the other hand, the 1e-1hh transition interacts only with the TE mode light, and not with the TM mode light.
As mentioned before, however, the switching operation accompanying the application of the electric field can be enabled by the excitonic absorption corresponding to the 1e-1hh transition.
Accordingly, the conventional optical switches with the rectangular quantum well structure cannot easily operate for the TM mode light, although they can satisfactorily operate for the TE mode light. Thus, the switching operation is dependent on polarization, and the TM mode light cannot be easily modulated.